Persistent: replace init method with a default value akin to those in PropField and Parameter.
Add features needed for first stage of porting A Nice Game of Chess.
Link flavor boilerplate: consolidate get_all_reverse method into reverse argument on get_all.
ScriptParamsLink: add get_all_by_data and get_one_by_data methods.
Message: write copy constructor; allow unspecialized message handlers.
GenericMessage, TimerMessage: add with_data static convenience methods.
ObjectProperty: rename add method to instantiate.
PropField: add instantiate and remove methods.
Persistent: cache value locally, switching from value to reference and pointer operators; add get_value helper method.
AI: add face_object method; make mode member non-const.
Add new nullptr-based "Empty" specialization of LGMulti, which can be constructed downstream.
Transition: new helper class for scripts causing gradual world changes, from KDScript's KDTransitionTrap.
Message: add get_data overload taking default value.